A level-readyAll subjects · Biology · WBI12 (Unit 2)
Cellulose, starch and plant fibres
Cellulose gives plant cell walls great strength. The arrangement of its fibres, and extra lignin, make xylem and sclerenchyma strong enough to use as fibres.
Starch and cellulose
Both are polymers of glucose, but:
- Starch is made of α-glucose (amylose and amylopectin). It is compact and insoluble: an energy store.
- Cellulose is made of β-glucose joined by 1,4-glycosidic bonds. Each glucose is rotated 180° relative to the next, so the chain is straight and unbranched. Many chains lie side by side, linked by many hydrogen bonds, forming microfibrils. Microfibrils group into larger fibres embedded in a matrix of other polysaccharides.
Strength in walls
In cell walls, microfibrils are laid down in layers, with each layer at an angle to the next. This makes the wall very strong in all directions (high tensile strength) while still flexible.
Secondary thickening: xylem vessels and sclerenchyma fibres lay down extra layers of cellulose and lignin. Lignin makes walls rigid and waterproof. This is why these cells make strong plant fibres, such as those from flax and hemp.
Comparing three tissues
You need to compare these three tissues by structure, position in the stem and function.
| Xylem vessels | Sclerenchyma fibres | Phloem sieve tubes | |
|---|---|---|---|
| Living? | Dead | Dead | Living (with companion cells) |
| Structure | Hollow tubes, no end walls, lignified, with pits | Long, narrow, tapering cells with heavily lignified walls | Sieve tube elements with sieve plates; no nucleus; not lignified |
| Function | Carry water and mineral ions up the plant; support | Support | Translocate sucrose and amino acids, up and down |
| Position in stem | Inner side of each vascular bundle | Cap on the outer side of each bundle | Outer side of each bundle, between xylem and sclerenchyma |
Worked example
Explain why cellulose has a high tensile strength.
- β-glucose gives straight, unbranched chains.
- Many hydrogen bonds link parallel chains into microfibrils.
- Microfibrils are layered at different angles in the wall.
Answer: Many hydrogen bonds between straight chains form strong microfibrils, laid in crossing layers.
Check yourself
- Which monomer makes up cellulose?
- α-glucose
- Fructose
- Amino acids
- β-glucose
Show answer
β-glucose. Cellulose is a polymer of β-glucose.
- What holds cellulose chains together in a microfibril?
- Peptide bonds
- Hydrogen bonds
- Ester bonds
- Disulfide bridges
Show answer
Hydrogen bonds. Many hydrogen bonds link parallel chains.
- Why are microfibrils laid down in layers at different angles?
- It makes the wall impermeable
- It stores energy
- It allows photosynthesis
- It gives strength in many directions
Show answer
It gives strength in many directions. Crossing layers resist forces from all directions.
- What is the effect of lignin in xylem walls?
- It makes the walls rigid and waterproof
- It makes the cells photosynthesise
- It dissolves cell walls
- It stores sugar
Show answer
It makes the walls rigid and waterproof. Lignin strengthens and waterproofs.
- Which tissue transports sucrose?
- Xylem
- Sclerenchyma
- Phloem
- Epidermis
Show answer
Phloem. Phloem translocates organic solutes.
- Which tissue is made of living cells?
- Phloem sieve tubes
- Xylem vessels
- Sclerenchyma fibres
- Lignified fibres
Show answer
Phloem sieve tubes. Sieve tube elements are living, though they have no nucleus.
- In a dicotyledonous stem vascular bundle, where is the sclerenchyma?
- In the centre of the stem
- On the inner side, next to the pith
- As a cap on the outer side of the bundle
- In the epidermis
Show answer
As a cap on the outer side of the bundle. Sclerenchyma caps the outside of each bundle, outside the phloem.
Sources
Last updated 7 November 2026. Spotted a mistake? Tell us.